Mobile devices, for example mobile phones or smart phones, are frequently used in connection with portable hands-free sets (PHF). These portable hands-free sets comprise for example a stereo earphone, a microphone and an antenna. The portable hands-free set is usually connected to the mobile device via a phone jack comprising for example four electrical contacts. However, currently there are at least two different audio connector standards used for these phone jacks, the OMPT (Open Mobile Terminal Platform) used in Europe and the CTIA (Cellular Telecommunications Industry Association) used in North America. The two connector types differ at least in an interchanged position of the ground and microphone contacts. Therefore, a circuit design is needed which automatically changes the interface to detect and support both standards. Such a circuit design requires the use of analogue semiconductor switches. These switches conduct high currents for the earphone in a common ground return path with the microphone. For this reason the switches must have a low on-resistance. However, switches having a lower on-resistance usually have a high capacitance which is furthermore non-linear. A non-linear circuit acts like an AM (Amplitude Modulation) demodulator. Further functionalities of the portable hands-free set like a FM (Frequency Modulation) radio antenna support requires high impedance ferrite beads which act like coils at lower frequencies. Together with the capacitance in the analogue semiconductor switches a resonance may be created. This may cause problems during mandatory type approval tests like current clamp tests and radiated immunity. Furthermore, this may cause audible bumblebee interference.
Therefore, there is a need to reduce a disturbance on a signal path of a semiconductor switch, especially a reduction of a resonance and demodulation.